Can you use phentermine as a suppository?

Umm, no. It is typically administered as a capsule that is swallowed. Unless your doctor prescribed it to be taken up the rectum.

Answer 2:

"Plugging" is a method of administration for some drugs (yes, even amongst straight men) that have a low oral bioavailability, such as morphine, to be dissolved in water and made in to an enema, for people who are scared of the needle. It is somewhat very dangerous in many ways.

Reference: experience of a recovering heroin addict who knew a person who "plugged" pills.